An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer (typically air) in a combustion chamber that is an integral part of the engine. Internal combustion engines are often employed for powering vehicles, either as a primary power source, or as part of a hybrid powertrain. In a reciprocating internal combustion engine, expansion of the high-temperature and high-pressure gases in a combustion chamber space at the top of the cylinder produced by the combustion applies direct force to the engine's piston. The combustion force applied to the piston subsequently acts through the engine's connecting rod to turn the engine's crankshaft.
A compression ratio is one of the fundamental specifications of an internal combustion engine. An internal combustion engine's compression ratio is a value that represents the ratio of the volume of the engine's combustion chamber from its largest capacity to its smallest capacity. In a reciprocating internal combustion engine the compression ratio is typically defined as the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. An internal combustion engine's compression ratio greatly influences the subject engine's torque output and its fuel efficiency.